Global warming is enhancing the mobilization of organic carbon (C) from Arctic soils into streams, where it can be mineralized to CO(2)and released to the atmosphere. Abiotic photo-oxidation might drive C mineralization, but this process has not been quantitatively integrated with biological processes that also influence CO(2)dynamics in aquatic ecosystems. We measured CO(2)concentrations and the isotopic composition of dissolved inorganic C (delta C-13(DIC)) at diel resolution in two Arctic streams, and coupled this with whole-system metabolism estimates to assess the effect of biotic and abiotic processes on stream C dynamics. CO(2)concentrations consistently decreased from night to day, a pattern counter to the hypothesis that photo-oxidation is the dominant source of CO2. Instead, the observed decrease in CO(2)during daytime was explained by photosynthetic rates, which were strongly correlated with diurnal changes in delta(13)C(DIC)values. However, on days when modeled photosynthetic rates were near zero, there was still a significant diel change in delta(13)C(DIC)values, suggesting that metabolic estimates are partly masked by O(2)consumption from photo-oxidation. Our results suggest that 6-12 mmol CO2-C m(-2)d(-1)may be generated from photo-oxidation, a range that corresponds well to previous laboratory measurements. Moreover, ecosystem respiration rates were 10 times greater than published photo-oxidation rates for these Arctic streams, and accounted for 33-80% of total CO(2)evasion. Our results suggest that metabolic activity is the dominant process for CO(2)production in Arctic streams. Thus, future aquatic CO(2)emissions may depend on how biotic processes respond to the ongoing environmental change.